A. Field of the Invention
The present invention relates to semiconductor apparatuses, and methods of manufacturing semiconductor apparatuses, and, in particular, semiconductor apparatuses such as power semiconductor devices.
B. Description of Related Art
The ignition coil for igniting an engine, for example, on an automobile, is provided with an ignitor. In the ignitor, a power semiconductor device is used. The power semiconductor device makes a current flow through a primary coil in the ignition coil and interrupts the current flowing through the primary coil to electronically control the ignition timing.
In a conventional ignitor, a hybrid integrated circuit (“IC”) is used. The hybrid IC includes, for obtaining higher functions, a ceramic baseboard, electric circuits formed on the ceramic baseboard by sintering an electrically conductive material, and electronic parts such as an IC and a capacitor mounted on the electric circuits. In the other conventional ignitor disclosed in Japanese Unexamined Patent Application Publication No. 2009-129952 (also referred to herein as “Patent Document 1”) a lead frame is divided into electrical wiring circuit sections, power semiconductor devices are mounted on the respective wiring circuit sections, and electronic parts are mounted on the wiring circuit sections such that the electronic part bridges the gap between the wiring circuit sections.
In the ignitor that employs a power semiconductor device such as an insulated-gate bipolar transistor (hereinafter referred to as an “IGBT”), it is necessary for the current path to exhibit a high heat capacity, since a high current flows through the IGBT, causing a high heating value. For meeting the demand for obtaining higher functions, it is necessary to mount many electronic parts on the ignitor package. However, if many electronic parts are mounted thereon while securing the required heat capacity, the ignitor package will be inevitably large in the external shape thereof. For solving the problem described above, Japanese Unexamined Patent Application Publication No. 2004-119676 (also referred to herein as “Patent Document 2”) discloses a technique that mounts electronic parts on both surfaces of a lead frame to reduce the substantial area, on which the electronic parts are mounted.
The power semiconductor device includes a first main terminal bonded directly to the parts mounting surface of a wiring circuit section. The power semiconductor device further includes a second main terminal and a control terminal connected to separate wiring circuit sections with wires. Generally, the wirings are conducted by directly bonding, for example, an aluminum wire to copper, that is the lead frame material, by ultrasonic bonding. In the bonding, a copper-aluminum alloy layer is formed on the bonding plane of copper and aluminum. If exposed to a high temperature, the copper-aluminum alloy layer will grow, causing a brittle layer. Therefore, the copper-aluminum alloy layer will hardly endures the thermal stress caused by temperature change and will be fractured. For the general countermeasures against the fracture, a plating of a metallic material containing nickel (Ni) as the main component thereof (hereinafter referred to as a “nickel plating” or a “Ni-plating”) is formed in advance in the area, in which the wire bonding to the lead frame will be conducted. The Ni-plating is formed to prevent defective bonding from causing in the bonding portion between the nickel wire and the copper lead frame.
Due to the manufacturing costs, the Ni-plating is formed not only in the area, in which the wire bonding to the lead frame is conducted, but also across the bonding areas in the direction, in which lead frames are arranged continuously, such that stripe-shaped Ni-plating films extend in parallel to each other. The Ni-plating described above poses the problems as described below.
Generally, the adhesiveness of the Ni-plating to the solder, the adhesiveness thereof to the electrically conductive adhesive, and the adhesiveness thereof to the mold resin are not so good. The area, in which the Ni-plating is formed, is used for the bonding area. However, the area, in which the Ni-plating is formed, is a useless area that can not be used for the area, to which electronic parts are bonded with a solder or with an electrically conductive adhesive. Therefore, the number of the electronic parts mountable on the lead frame will be limited.
Since the stripe-shaped plating should be formed so as not to cover the area, on which electronic parts are mounted, the number of the bonding areas with wires is also limited. Due to the poor adhesiveness of the Ni-plating to the mold resin, the difference between the thermal expansion coefficients of the mold resin and the lead frame exerts thermal stresses repeatedly to the wires in the heat cycle caused by the heat generation from the power semiconductor device, finally cutting the wires in the vicinity of the bonded portions thereof.
In the case in which electronic parts are mounted on both surfaces of the lead frame to prevent the package external shape from enlarging, the electronic parts may be displaced from the predetermined positions thereof, because it is necessary to tilt or turn over the lead frame in the manufacturing process. For preventing the electronic parts from being displaced from the predetermined positions thereof, the electronic parts on one lead frame surface are held temporarily with an insulating film or an insulating resin. Then, the other electronic parts are mounted on the other lead frame surface and connected to the other lead frame surface electrically by means of melting a solder and such a bonding material by the reflow technique and such a technique. The technique that holds the electronic parts temporarily on one lead frame surface is not applicable to the electronic parts which bridge the gap between the divided wiring circuit sections, since spaces are caused under the electronic parts and since it is impossible to stick any film thereto. Since the technique that holds the electronic parts temporarily with a resin causes many additional steps, the technique that holds the electronic parts temporarily with a resin is not applicable.
Accordingly, there is a need in the art for a method of manufacturing a semiconductor apparatus that facilitates mounting semiconductor devices and electronic parts on both surfaces of a lead frame divided to form wiring circuits without through complicated manufacturing steps.